1. Field of the Invention
The present invention relates to an optical pickup device which carries out recording/reproducing of information signals from an information recording media, such as optical disc.
2. Description of the Related Art
There have been conventionally utilized optical discs as household information recording media, for example, so-called “CD (Compact Disc)”, “DVD (Digital Versatile Disc)”, and others in widespread use. Not only playback-only standard on initial use but also recordable standards (e.g. “CD-R”, “CD-RW”, “DVD-RAM”, “DVD-R”, “DVD-RW”, “+R”, “+RW”) have recently become familiar rapidly. Additionally, “BD (Blu-ray Disc)” and “HD-DVD” are also coming into practical use, as optical discs dealing with high definition video signals.
In an optical pickup device forming a main component of a read-write apparatus (i.e. an optical disc system) performing recording/reproducing of information in relation to such an optical disc, a light receiving/emitting function is fundamental. Additionally, for both focus-servo (adjustment in focusing) and tracking-servo (adjustment in tracking) operations being physical basic actions in the read-write apparatus, optical and electrical functions to detect a focus-error signal and a tracking-error signal are essential to the optical pickup device.
In order to deal with a variety of optical discs of the above-mentioned standards, an optical pickup device is required to have some error-signal detecting functions suitable to respective optical discs of such standards. Further, it is expected to develop an optical pickup device which has not only these functions but a reasonable optical system allowing its structure to be simplified and also allowing information signals to be picked up with high accuracy. In fact, the development of an optical pickup device mentioned above is a competitive issue to various companies in this industry.
In a read-write apparatus, generally, signal outputs picked up by the optical pickup device are supplied to a FEP (Front End Processor) having integral circuits (IC) for performing both calculations and drive controls based on the signal outputs. In this view, it is desirable that an optical pickup device as one component of the read-write apparatus is capable of outputting all-purpose signals requiring neither special calculation nor development of an exclusive FEP.
As one detecting method for detecting a focus-error signal in a pickup device in related art, Japanese Patent Laid-open Publication No. H10(1998)-64080 discloses so-called “astigmatism method” (see FIGS. 1, 2 and 3 in the publication). By way of example, FIG. 1 shows one optical pickup device adopting this “astigmatism method”. In this pickup device, a laser beam is radiated from a semiconductor laser 101. A collimator lens 102 changes the radiated laser beam to a parallel laser beam. Then, the so-directional laser beam is transmitted to an objective lens 104 through a polarizing beam splitter 103. The laser beam is converged by the objective lens 104 and further emitted (or projected) to an optical disc 105. Then, the laser beam forms a light spot on a recording track (pit line) on a recording surface of the optical disc 105. Meanwhile, a reflection light from the optical disc 105 is transmitted to the polarizing beam splitter 103 through the objective lens 104. Then, the reflection light is further reflected toward a detection lens 107 by the polarizing beam splitter 103. In succession, the reflection light is converged by the detection lens 107 and transmitted through a cylindrical lens 108 as an astigmatism generator. As shown in FIG. 2B, the reflection light via the cylindrical lens 108 reaches a quadrant photo detector 109 and forms a spot SP in the vicinity of a center of a light receiving surface of the detector 109. The photo detector 109 has four light receiving areas DET1, DET2, DET3 and DET4 obtained by dividing the light receiving surface into four zones by two orthogonal-oriented parting lines L1, L2. In the quadrant photo detector 109, the parting line L1 is parallel to a tangential direction in optical mapping, which is substantially identical to a tangential line of the recording track of the optical disc 105. On the other hand, the parting line L2 is parallel to a radial direction in optical mapping, which is substantial identical to a radial direction of the optical disc 105.
The cylindrical lens 108 is provided to produce an astigmatism in the reflection light from the photo disc 105. In connection with this reflection light, there are produced, at different positions on its optical axis, two focal lines which intersect with each other at right angles. When the laser beam projected onto the signal recording surface of the optical disc 105 is in an in-focus state, the spot SP on the quadrant photo detector 109 is positioned between the above-mentioned two focal lines and also shaped to be substantially circular (perfect circle), as shown in FIG. 2B.
If the focal point of the laser beam emitted to the photo disc 105 is on the front side of the signal recording surface (i.e. when a distance between the optical disc 105 and the objective lens 104 gets larger), then the spot SP to be formed on the detector 109 by the reflection light is actually formed in a position close to one of the focal lines, providing an oblong spot having a long axis in one diagonal direction of the detector 109, as shown in FIG. 2A.
If the focal point of the laser beam emitted to the photo disc 105 is on the back side of the signal recording surface (i.e. when a distance between the optical disc 105 and the objective lens 104 gets smaller), then the spot SP to be formed on the detector 109 by the reflection light is actually formed in a position close to the other one of the focal lines, providing an oblong spot having a long axis in the other diagonal direction of the detector 109, as shown in FIG. 2C.
The quadrant photo detector 109 generates four electric signals proportional to respective intensities of lights projected on the light receiving areas DET1, DET2, DET3 and DET4 and supplies a focus-error detecting circuit 110 with these signals, as shown in FIG. 1. On receipt of the signals, the focus-error detecting circuit 110 generates a focus-error signal (FES) and supplies it to an actuator driving circuit 111. Then, the actuator driving circuit 111 supplies an actuator 112 for carrying the objective lens 104 with a focusing drive signal. On receipt of the focusing drive signal, the actuator 112 operates to move the objective lens 104 in the direction of the optical axis. The focus-error detecting circuit 110 is connected to the light receiving areas DET1, DET2, DET3 and DET4 of the quadrant photo detector 109, as shown in FIG. 3. In the focus-error detecting circuit 110, light-to-photocurrent converted outputs from two light receiving areas DET1 and DET3, which are symmetric about a center O of the light receiving surface of the photo detector 109, are added to each other by a first accumulator 113, while light-to-photocurrent converted outputs from two light receiving areas DET2 and DET4 are added to each other by a second accumulator 114. Both outputs from these accumulators 113, 114 are supplied to a differential amplifier 115. The differential amplifier 115 calculates a difference between the outputs on supply and outputs the difference in the form of a focus-error signal (FES). That is, if representing respective output signals from the light receiving areas by signs “DET1”, “DET2”, “DET3” and “DET4”, then the focus-error signal FES is obtained by the expression: FES =(DET1+DET3)−(DET2+DET4).
When the laser beam emitted to the signal recording surface of the optical disc 105 is in the in-focus state, the light intensity of the spot SP on the quadrant photo detector 109 exhibits a symmetrical intensity distribution about the center O of the light receiving surface, so that the focus-error signal becomes zero (0). While, if the laser beam emitted to the signal recording surface of the optical disc 105 is not in the in-focus state, them a sum of light-to-photocurrent converted outputs from two light receiving areas on one diagonal line of the detector 109 differs from that of light-to-photocurrent converted outputs from the other light receiving areas on the other diagonal line of the detector 109. Thus, in this case, the focus error signal outputted from the differential amplifier 115 has a value corresponding to a focus error at that time.
As another detecting method for detecting a focus-error signal in the pickup device in related art, Japanese Patent Publication Nos. 2629781 (see FIG. 8) and 2724422 (see FIG. 6) disclose so-called “SSD (spot size detection) method”. In a pickup device employing this spot size method, as shown in FIG. 4, a reflection light from an optical disc is branched to two or more light fluxes R1, R2 by a not-shown hologram element or the like. Then, the light fluxes R1, R2 are further accompanied with different positive powers to be convergent lights, respectively. The light flux R1 is received by a light receiving element 201 before reaching a flux's converging point, while the other light flux R2 is received by another light receiving element 202 after passing through a flux's converging point.
In the light receiving element 201, its light receiving surface is divided into three light receiving areas DET1, DET2 and DET3 by parallel parting lines, as shown in FIG. 5. While, in the other light receiving element 202, its light receiving surface is divided into three light receiving areas DET4, DET5 and DET6 by parallel parting lines. The respective light fluxes R1, R2 form spots in the vicinity of respective centers of the light receiving surfaces.
When the laser beam emitted to the signal recording surface of the optical disc is in the in-focus state, the spots that the light fluxes R1, R2 branched from the reflection light form on the light receiving elements 201, 202 have sizes substantially equal to each other, as shown in FIG. 5.
If the focal point of the laser beam emitted to the optical disc 105 is on the back side of the signal recording surface (i.e. when a distance between the optical disc 105 and the objective lens 104 gets smaller), then the spots that the light fluxes R1, R2 branched from the reflection light form on the light receiving elements 201, 202 are formed in the vicinity of a converging point of the light flux R1, so that one spot in charge of the light flux R1 gets smaller, while the other spot in charge of the light flux R2 gets larger.
If the focal point of the laser beam emitted to the optical disc is on the front side of the signal recording surface (i.e. when a distance between the optical disc and the objective lens gets larger), then the spots that the light fluxes R1, R2 branched from the reflection light form on the light receiving elements 201, 202 are formed in the vicinity of a converging point of the other light flux R2, so that one spot in charge of the light flux R1 gets larger, while the other spot in charge of the light flux R2 gets smaller. If the spots formed by the light fluxes R1, R2 get smaller, then flux powers concentrate on the light receiving areas DET2, DET5 at respective centers of the light receiving elements 201, 202. Conversely, if the spots formed by the light fluxes R1, R2 get larger, then flux powers are diffused to the light receiving areas DET1, DET3, DET4 and DET6 on both sides of the light receiving elements 201, 202. That is, if representing respective output signals from the light receiving areas 201, 202 by signs “DET1”, “DET2”, “DET3”, “DET4”, “DET5” and “DET6” then the focus-error signal FES is obtained by the expression: FES=(DET1+DET3+DET5)−(DET2+DET4+DET6).
In the above-mentioned optical pickup devices for “CD” and “DVD”, meanwhile, it is often the case that the astigmatism method is adopted in an optical pickup device (in bulk optical system) having its optical constitution in the form of non-integration. Additionally, in case of the astigmatism method, it is necessary to adjust the position of a quadrant photo detector with respect to the reflection light from an optical disc with high accuracy, causing both manufacturing and adjusting processes of the pickup device to be tangled.
Nevertheless, the astigmatism method would be advantageous if adopting so-called “push-pull method” or “DPD (differential phase detection) method” as a method for detecting a tracking-error signal, because the intensity of a reflection light from an optical disc is separated into four quadrants by both tangential and radial axes and represented in the form of four components in the astigmatism method. Thus, signal outputs from a quadrant photo detector in the astigmatism method could be utilized as they are in case of adopting either “push-pull method” or “DPD (differential phase detection) method”.
On the other hand, it is often the case that the SSD method is adopted in an optical pickup device having an integrated optical system using a hologram element or the like. That is, in case of adopting the SSD method, different convex-lens powers (positive powers) are appended to two light fluxes (i.e. +first-order diffraction lights by the hologram element) and furthermore, each intensity of these (two) light fluxes is detected in the form of respective information from three divisional areas obtained by dividing one light receiving area by parting lines in parallel with the radial axis. Therefore, in the optical pickup device adopting the SSD method, there is no possibility that the detecting accuracy for the focus-error signal is influenced by either light spots' moving due to variations in diffraction angles at the hologram depending on wavelengths of the light fluxes element or optical system's assembling error in a direction of the radial axis. After all, the optical pickup device adopting the SSD method is capable of loosening its assembling accuracy in the direction of the radial axis.
Additionally, since two light fluxes used in the SSD method is symmetrical to each other about the tangential direction and further accompanied with different lens powers, it is also possible to loosen the assembling accuracy with respect to the tangential direction complementarily.
However, in case of adopting the SSD method, it is difficult to detect intensity components of the reflection light from an optical disc, which are divided by parting lines in an equivalent direction of the tangential axis and also required in both the DPD method and the push-pull method. Even if applying such a division to a hologram element, it is necessary to divide another element's area different from areas of tripartite division for the SSD method since there is no agreement in logic between the quadrant division (for DPD, push-pull) using both radial and tangential axes and the tripartite division for the SSD method.
Therefore, if adopting either the push-pull method or the DPD method in order to detect a tracking-error signal, then an optical path for detecting the tracking-error signal has to be separated from an optical path for detecting a focus-error signal as well as their respective output systems, causing the optical pickup device to be complicated in structure.
In this way, there are good and bad points in both the astigmatism method and the SSD method. In either case of adopting one method, an exclusive selection has been required by the sacrifice of advantages of the other method.
Still further, since there is a difference in an arithmetic logic of the focus-error signal between the astigmatism method and the SSD method, a FEP (Front End Processor) for generating a focus-error signal for the astigmatism method is incompatible with another FEP for the SSD method, requiring an exclusive FEP with respect to each method. Thus, for one read-write apparatus that has already come into existence as one unit for the astigmatism method, an optical pickup device in accordance with the SSD method could not have any interchangeable component. Further, a FEP addressing both of the methods would be complicated in structure.